Think about pushing a book slowly across a long table..If you stopped pushing it most likely it would stop moving without you exerting force upon it. Now think about picking the book up slowly until it is over your head and then letting go..will it continue to move after you let it go? YES!!! If both situations, you are exerting force upon an object and then removing the force..but in one situation there is continued movement and one there is not..WHY?

 According to Newton’s first law of motion, the book’s motion changes ONLY if an unbalanced force acts on it…  When book one slides across the table, the force of FRICTION causes the book to stop.  When book two falls, the force of GRAVITY causes it to accelerate downward.

When you pushed the book across the table, the surface of the book rubs against the surface of the table. Even if an object looks smooth, it can still have things that we cannot see that can rub against other surfaces and cause the object to decelerate. Friction: the force that one surface exerts on another when the two rub together.

Friction acts in a direction opposite to the object’s direction of motion. Without friction, an object that was pushed or pulled would continue at a constant speed forever. The strength of the force of friction depends on two factors: the types of surfaces involved and how hard the surfaces push together. Rough surfaces produce a greater friction than smooth surfaces.

 Rough surfaces produce a greater friction than smooth surfaces.  If you tried to sled on snow and sled on sand…on which surface would you go faster??  THE SNOW because it is a much smoother surface.

 You are able to walk because of the friction between the floor and the soles of your shoes. Without friction you would only slide across the floor and you would never move forward.  An automobile moves because of friction between its tires and the road.  Ballet dancers spread a powder on the soles of their shoes so they don’t slip on the floor.

 There are three different types of friction: sliding friction, rolling friction, and fluid friction.  Sliding friction: when objects slide over a surface. (ex.skiing)  Rolling friction: when an object rolls over another object. (rolling pin)  Fluid Friction: an object moves through a liquid or gas. (swimming)

 The force required to overcome ROLLING friction is much less than the force needed to overcome SLIDING friction.  The force need to overcome FLUID friction is usually less than that needed to overcome SLIDING friction.

 Friction explains why a book comes to a stop after it is pushed…but why does the same book fall to the ground if you lift it and let it go??  Newton realized that a force acts to pull an object straight down toward the Earth.  He called this force gravity!!  Gravity: the force that pulls objects toward each other.

 When the only force acting upon an object is gravity, it is said to be in FREE FALL.  An object in free fall accelerates as it falls.  In a free fall, the force of gravity is an unbalanced force, and an unbalanced force cause an object to accelerate.  Even though it seems hard to believe, ALL objects in free fall accelerate at the same rate regardless of mass!!!

 Have you ever played catch with someone?  If so, you have used PROJECTILE MOTION.  Will a projectile object land on the ground at the same time as an object that is dropped down??  An object that is dropped and a object that a thrown from one side to another are BOTH in free fall and will hit the ground at the same time.

 Even though all objects are SUPPOSED to fall at the same rate..sometimes that is not the case.  Objects falling through the air experience a type of fluid friction called air resistance.  Air resistance is NOT the same for all objects.  The larger the mass of the object, the greater the air resistance.  Example: a leaf falls more slowly than an acorn because it is wider and has a greater mass.  The greatest velocity an object can reach is known as TERMINAL VELOCITY.

 Weight: the force of gravity on a person or object at the surface of a planet.  Formula for weight: Mass X Acceleration due to gravity.  Acceleration due to gravity = 9.8 m/s ^ 2  Example: A 50kg person weighs… 50 kg X 9.8 m/s ^2 = 490 N

 The law of universal gravitation states that the force of gravity acts between all objects in the universe.  Any two objects in the universe attract each other.  Earth and other objects around you all attract you and each other.